Heliostat Assignment in a Multi-Tower Field

ABSTRACT

Methods, systems, and apparatus, including computer programs encoded on one or more computer storage devices, for collecting solar energy using heliostats arranged about a collection of solar energy receivers. For each heliostat, estimated efficiencies of the heliostat in directing solar rays to two or more receivers at various times of day are determined. Each heliostat is assigned to direct solar rays to two or more different receivers at two or more different times of day, wherein each heliostat directs solar rays to one receiver at a time and the assigning is based on the determined estimated efficiencies for the heliostat at the various of times of day. In some implementations, the receivers are repositionable.

TECHNICAL FIELD

This specification relates to heliostat assignment in a multi-tower field of heliostats.

BACKGROUND

Heliostats can be used to collect radiation from the Sun. Specifically, a heliostat can include one or more mirrors to direct solar rays toward a receiver mounted on a receiver tower. Some types of heliostats are capable of moving their one or more reflective surfaces, i.e., mirrors, as the Sun moves across the sky, both throughout the day and over the course of the year, in order to more efficiently direct solar rays to the receiver. Solar rays that are directed to the receiver can then be used to generate solar power. A field of heliostats can be placed surrounding one or more receivers to increase the quantity of radiation collected and optimize the amount of solar power that is generated. The solar power is converted to electricity by either the receiver or a generator that is coupled to the receiver.

SUMMARY

In general, one innovative aspect of the subject matter described in this specification can be embodied in methods that include the following. For each heliostat, of multiple heliostats arranged about multiple receiver towers, where each receiver tower has a receiver mounted to the tower that is configured to receive solar rays reflected from, multiple estimated efficiencies of the heliostat in directing solar rays to two or more receiver towers at multiple times of day are determined. Each heliostat is assigned to direct solar rays to two or more different receiver towers at two or more different times of day, wherein each heliostat directs solar rays to one receiver tower at a time and the assigning is based on the determined estimated efficiencies for the heliostat at the multiple times of day.

These and other embodiments can each optionally include one or more of the following features. Determining multiple estimated efficiencies of each heliostat may further include determining estimated efficiencies of the heliostat in directing solar rays to the two or more receiver towers at a multiple times of day on multiple days of a year, and assigning each heliostat to direct solar rays to two or more different receiver towers is further based on the determined estimated efficiencies. The assignment of each heliostat to direct solar rays to two or more different receiver towers may include assigning each heliostat to direct solar rays to two or more different receiver towers that are each positioned south of the heliostat for a heliostat field in the northern hemisphere and that are each positioned north of the heliostat for a heliostat field in the southern hemisphere. For each heliostat, the position of one or more reflective surfaces included on the heliostat may be controlled based on a position of the Sun and which receiver tower the heliostat is assigned to direct solar rays toward.

The cost and the benefit of reassigning a heliostat from one receiver to another can be determined, and the if the benefit outweighs the cost, then the heliostat can be reassigned, otherwise the heliostat assignment can remain unchanged. Flux distribution over a surface of two or more receivers when a particular heliostat is assigned to direct solar rays to them can be determined, and the heliostat assignment can be based on this determination, such that flux distribution over a receiver surface can be managed.

In another aspect, a method for operating a multi-tower heliostat field includes, for each receiver tower of multiple receiver towers about which are arranged multiple heliostats, assigning a set of heliostats to reflect solar rays to the receiver tower. Each receiver tower has a receiver mounted to the tower that is configured to receive solar rays reflected from the assigned set of heliostats. For each receiver tower, based on a level of solar energy absorbed by the receiver mounted to the tower, reassigning which heliostats are included in the set of heliostats that are assigned to direct solar rays to the receiver tower.

These and other embodiments can each optionally include one or more of the following features. The level of solar energy absorbed by the receiver mounted to the tower during daylight hours may be monitored for each receiver. When the monitored level of solar energy drops below a first predetermined threshold value for one or more of the receiver towers, a subset of the receiver towers may be closed and included in the sets of heliostats assigned to direct solar rays to the closed-in subset of receiver towers may be reassigned to different receiver towers included in the plurality of receiver towers. When the monitored level of solar energy rises above a second predetermined threshold value for one or more of the receiver towers, one or more of the closed-in receiver towers may be reactivated and at least some of the heliostats may be reassigned to direct solar rays to the reactivated one or more receiver towers. For each heliostat, the positioning of one or more reflective surfaces included on the heliostat may be controlled based on a position of the Sun and which receiver tower the heliostat is assigned to direct solar rays toward.

In another aspect, a method for operating a multi-tower heliostat field includes, for each receiver tower of multiple receiver towers about which are arranged multiple heliostats, a set of heliostats are assigned to direct solar rays to the receiver tower. Each receiver tower has a receiver mounted to the tower that is configured to receive solar rays reflected from the assigned set of heliostats. Based on estimated levels of solar ray intensity at different times of the day, a subset of the receiver towers are closed in during one or more time periods a day and the heliostats included in the sets of heliostats assigned to the closed-in subset of receiver towers are reassigned to different receiver towers during those time periods.

These and other embodiments can each optionally include one or more of the following features. Closing in a subset of the receiver towers and reassigning the heliostats may be based on estimated levels of solar ray intensity at different times of the day and at different times of the year. For each heliostat, positioning of one or more reflective surfaces included on the heliostat may be controlled based on a position of the Sun and which receiver tower the heliostat is assigned to direct solar rays toward.

In another aspect, a heliostat field system includes multiple heliostats, and multiple receiver towers, where each receiver tower has a receiver mounted to the tower that is configured to receive solar rays reflected from a set of heliostats. The set of heliostats are assigned to direct solar rays to the receiver tower. The system further includes a assignment control system configured to assign each heliostat to direct solar rays to two or more different receiver towers at two or more different times of day. Each heliostat directs solar rays to one receiver tower at a time and the assigning is based on estimated efficiencies of the heliostat in directing solar rays to the two or more receiver towers determined for a multiple times of day.

These and other embodiments can each optionally include one or more of the following features. The heliostat field may also include a heliostat tracking control system configured to control, for each of the heliostats, positioning of one or more reflective surfaces included on each heliostat based on a position of the Sun and which receiver tower the heliostat is assigned to direct solar rays toward. The assignment control system may be further configured to assign the heliostats to direct solar rays to a subset of the receiver towers, such that the remaining receiver towers are closed-in, based on estimated levels of solar ray intensity at different times of the day. The assignment control system may be further configured to assign the heliostats to direct solar rays to the receiver towers, such that the closed-in receiver towers are reactivated, based on the estimated levels of solar ray intensity at different times of the day.

A solar energy level monitoring system may be configured to monitor, for each of at least some of the receiver towers, a level of solar energy absorbed by the receiver mounted on the receiver tower. The assignment control system can be further configured to assign each heliostat to direct solar rays to two or more different receiver towers based on the monitored levels of solar energy. The assignment control system may be further configured to assign the heliostats to direct solar rays to a subset of the receiver towers, such that the remaining receiver towers are closed-in, when at least some of the monitored levels of solar energy are below a predetermined first threshold value. The assignment control system may be further configured to assign the heliostats to direct solar rays to the receiver towers, such that the closed-in receiver towers are reactivated, when at least some of the monitored levels of solar energy are above a predetermined second threshold value.

Other embodiments of the above described aspects include corresponding systems, apparatus, and computer programs, configured to perform the actions of the methods, encoded on computer storage devices.

Particular embodiments of the subject matter described in this specification can be implemented so as to realize one or more of the following advantages. The solar energy collection of a solar energy collection facility that includes multiple heliostats assigned to direct solar rays to multiple receivers can be improved. Cosine losses can be reduced, thereby improving the efficiencies of the heliostats in directing solar rays to receivers. The efficiencies of receivers can be improved, for example, by closing-in some receivers for certain portions of a day, therefore improving the efficiency of the activated receivers and the overall efficiency of the solar energy collection facility. If a particular receiver is over heated or inactive for maintenance or otherwise, heliostats that may have typically been assigned to the receiver can be assigned to neighboring receivers thereby making better use of the neighboring receivers and extending their number of productive hours in a day. Flux distribution over a surface of a receiver can be managed, so as to optimize a temperature of a working fluid receiving solar heat from the receiver. The efficiency of a solar energy collection facility can be optimized by determining whether the benefit of re-assigning a heliostat from one receiver to another outweighs the cost of the reassignment.

The details of one or more embodiments of the subject matter described in this specification are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C illustrate examples of the cosine effect for two heliostats located on opposite sides of a solar energy receiver in a solar energy collection facility.

FIG. 2 is an example of a fixed assignment of heliostats and solar energy receiver towers in a solar energy collection facility.

FIGS. 3A-3C illustrate example configurations of a solar energy collection facility 300 that improve solar energy collection efficiency.

FIG. 4 is a schematic representation of the Sun's position in the sky throughout an example day.

FIGS. 5A-5C show example configurations of heliostats to concentrate available solar energy under varying daylight conditions.

FIG. 6 is a flow diagram of an example process for assigning and reassigning a collection of heliostats among a collection of solar energy receiver towers in a solar energy collection facility.

FIGS. 7A-7B illustrate example configurations of heliostats and solar energy receiver towers in all north field configuration.

FIG. 8 is a block diagram of an example heliostat field system.

FIG. 9 is a schematic representation of an example receiver mounted on a receiver tower in a heliostat field.

FIG. 10 is a schematic diagram of an example of a generic computer system.

Like reference numbers and designations in the various drawings indicate like elements.

DETAILED DESCRIPTION

A heliostat is assigned to direct solar rays toward a receiver that is typically mounted on a receiver tower. Various factors can affect the efficiency of the heliostat in directing the solar rays to the receiver, including the position of the Sun relative to the heliostat's reflective surface or surfaces, weather, and environmental conditions affecting Sun intensity. In terms of the position of the Sun relative to the heliostat's reflective surface(s), the cosine effect represents the difference between the amount of energy falling on a surface pointing at the Sun, and a surface parallel to the surface of the earth. As this concept applies to heliostats and other types of solar reflectors, a heliostat generally reflects the greatest amount of solar energy when the plane of the heliostat's reflective surface is oriented substantially perpendicular to the incoming rays of sunlight. However, since the purpose of a heliostat in a solar energy collection scenario is generally to change angles so as to reflect incoming sunlight to a solar energy collector, a perpendicular orientation relative to incoming sunlight is rarely, if ever, useful. As the heliostat is angled away from perpendicular, the effective amount of surface area (and therefore also the amount of reflected solar energy) decreases.

One factor for determining an optimum heliostat field layout is the cosine efficiency of the heliostat. In some implementations, this efficiency can depend on the Sun's position, the location of the individual heliostat relative to the receiver tower, or a combination of both. The heliostat is positioned by the tracking mechanism so that its surface normal bisects the angle between the Sun's rays and a line from the heliostat to the tower. The effective reflection area of the heliostat is reduced by the cosine of one-half of this angle (e.g., angle θ_(i) shown in FIG. 1A). This may be visualized by considering heliostats at two positions in a field as shown in FIGS. 1A-1C.

FIGS. 1A-1C illustrate examples of the cosine effect for two heliostats 110 and 120 located on opposite sides of a solar energy receiver to 125 in a solar energy collection facility 100. A solar energy receiver 130 is mounted atop the receiver tower 125 to receive solar energy reflected by the heliostats 110 and 120. To simplify the example, the facility 100 is shown in a substantially noontime configuration (e.g., the Sun 102 is at its peak south of the facility 100).

The heliostat 110 has a small cosine loss (e.g., compared to the heliostat 120) since its surface normal is almost pointing toward the receiver 130, resulting in an effective reflective area 112. The heliostat 120 has a larger cosine loss and a smaller effective reflective area 122 because of the position it must assume in order to reflect the Sun's 102 rays onto the receiver 130. It should be noted that in some implementations, the most efficient heliostats may be located opposite the Sun 102 relative to the receiver 130.

In the illustrated example, the heliostat 120 is located in a south field (e.g., south of the solar energy receiver 130), and the heliostat 110 is located in a north field (e.g., north of the solar energy receiver tower 130). Generally speaking, in the northern hemisphere the Sun 102 appears to move across the sky in an arc that is to the south of overhead. In the examples used throughout this document, the terms north field and south field will generally be given in relation, but are not limited to, implementations in the northern hemisphere where the Sun's path is southward of overhead. It should be noted that any of the examples given in this document can also be applied to southern hemispheric applications where the Sun's path is generally north of overhead. In such implementations, the terms “north” field and “south” field can be transposed to implement functionality similar to that described for northern hemispheric implementations.

An expression for calculation of the cosine of this half angle has been developed as the following equation:

${\cos \; 2\; \theta_{i}} = \frac{{\left( {z_{0} - z_{1}} \right)\sin \; \alpha}\; - {e_{1}\cos \; \alpha \; \sin \; A} - {n_{1}\cos \; \alpha \; \cos \; A}}{\left\lbrack {\left( {z_{0} - z_{1}} \right)^{2} + e_{1}^{2} + n_{1}^{2}} \right\rbrack^{1/2}}$

Where α and A are the Sun's altitude and azimuth angles, respectively, and z, e, and n are the orthogonal coordinates from a point on the receiver 130 at the height of the heliostats' 110 and 120 mirrors. An angle 114 represented by θ_(i) is the angle between a Sun ray 116 and a surface normal 118. Similarly, an angle 114 also represented by θ_(i) is the angle between a Sun ray 116 and a surface normal 118.

FIG. 2 is an example of a fixed (i.e., static) assignment of heliostats and solar energy receiver in a solar energy collection facility 200. In the illustrated example, a collection of heliostats 210 is grouped into a collection of north fields 230 a-230 c and a collection of south fields 240 a-240 c. Each of the north fields 230 a-203 c and each of the south fields 240 a-240 c are permanently assigned to direct solar rays to one of the receivers 220 a-220 c. For example, the north field 230 a and the south field 240 a are assigned to direct solar rays to the receiver 220 a. Similarly, the north field 230 b and the south field 240 b are assigned to the receiver 220 b, just as the north field 230 c and the south field 240 c are assigned to the receiver 220 c. When a heliostat is assigned to a receiver, reflective surfaces on the heliostat are positioned to direct solar rays to the receiver. In some examples, the reflective surfaces move throughout the course of a day to track the movement of the Sun across the sky.

Equation 1 can be used to show that for northern hemispheric implementations, heliostats located opposite the Sun relative to the receiver can be the most efficient. In the example of the facility 200, the heliostats 210 in the north fields 230 a-230 c can generally have lower cosine losses than the heliostats 210 in the south fields 240 a-240 c. In the morning, the heliostats 210 located west of their associated receivers 220 a-220 c will generally have higher efficiencies than those located east of the receivers 220 a-220 c. The opposite occurs in the afternoon, giving the east and west heliostats 210 an average efficiency in between the high and the low.

FIGS. 3A-3C illustrate example configurations of a solar energy collection facility 300 that improve solar energy collection efficiency. In particular, these configurations can reduce the cosine effect on a collection of heliostats 302 as the Sun moves across the sky. Sunlight shines on the facility 300 in a direction generally indicated by arrows 310. As such, FIG. 3A represents the facility 300 as it generally is configured in the morning (e.g., the Sun is shining out of the southeast). Likewise, FIG. 3B represents the facility 300 as it generally is configured around noon (e.g., the Sun is approximately due south) and FIG. 3C represents the facility 300 in the afternoon (e.g., the Sun is shining out of the southwest).

Referring to the morning configuration illustrated by FIG. 3A, the heliostat assignments are depicted by boundaries shown in broken lines. In the example shown, the boundaries 304 a-304 d follow paths roughly parallel to the Sun's rays, as indicated by the arrows 310. As such, the heliostats 302 between the boundaries 304 a and 304 b form a heliostat grouping 320 a that is roughly aligned along the direction of the Sun's rays and are assigned to reflect solar energy toward a solar energy receiver 330 a. The approximate direction of the reflected solar energy is represented by arrows 350. Similarly, a heliostat field 320 b and a heliostat field 320 c are also organized roughly parallel to the Sun's rays to reflect solar energy toward solar energy receivers 330 b and 330 c respectively.

As the Earth rotates, the Sun appears to move across the morning sky and reaches its solar apex around noon. Referring now to FIG. 3B, the heliostats 302 between boundaries 304 e and 304 f form a heliostat grouping 320 d that is assigned to the receiver 330 a, i.e., solar energy shining upon the grouping 320 d is reflected to the receiver 330 a. Similarly, the heliostat grouping 320 e is between boundaries 304 f and 304 g and assigned to reflect sunlight toward the receiver 330 b, and the heliostat grouping 320 f is between boundaries 304 g and 304 h and assigned to reflect sunlight toward the receiver 330 c. As such, the heliostat groupings 320 d-320 f assigned to the receivers 330 a-c at mid-day are different than the heliostat groupings assigned to the receivers 330 a-c in the morning. The heliostat assignments can change throughout the course of the day such the boundaries roughly align to the incoming solar radiation and therefore improve the overall cosine efficiency of the facility.

Referring now to FIG. 3C, the heliostats 302 between boundaries 304 i and 304 j form a heliostat grouping 320 g. Sunlight falling upon the grouping 304 g is reflected to the receiver 330 a. Similarly, the heliostat grouping 320 h between boundaries 304 j and 304 k is assigned to reflect sunlight toward the receiver 330 b, and the heliostat grouping 320 i between boundaries 304 k and 3041 is assigned to reflect sunlight toward the receiver 330 c. As such, the heliostat groupings 320 g-320 i utilize different groupings of the heliostats 302 compared to the groupings illustrated in FIGS. 3A and 3B to roughly align the groupings 320 g-320 i parallel to the incoming solar radiation and therefore improve the overall cosine efficiency of the facility in the afternoon. In the particular example shown, three different heliostat assignments are depicted in the morning, mid-day and afternoon. However, it should be understood that the assignments of the heliostats to the receivers can be changed at many times during the day, and the example shown is simplified for illustrative purposes.

FIG. 4 is a schematic representation of the Sun's position in the sky throughout an example day. In particular, FIG. 4 can be used to illustrate an example of how the Sun's position can influence the assignment of heliostats in a solar energy collection facility, such as the facility 100 of FIG. 1A or the facility 300 of FIGS. 3A-3C. An individual configuration of heliostats generally has a high cosine efficiency at one time of the day, and gradually diminishing efficiencies on either side of that time.

In some implementations, such as those illustrated by FIGS. 3A-3C, a solar energy collection facility may create multiple peak cosine efficiency times by varying how heliostats are assigned to receiver towers. Two, three, four, or more different configurations of heliostats and receivers may be defined and implemented for different periods of the day. For example, one configuration may have a peak cosine efficiency at mid-morning, another configuration may have a peak cosine efficiency at noon, and another configuration may have a peak efficiency at mid-afternoon. In some implementation, between the peak efficiency times may be times at which the cosine efficiencies of two timewise adjacent configurations may be substantially equal. For example, a morning configuration that had a peak cosine efficiency around 7 am and a mid-day configuration that peaks around 12 pm may have substantially equal cosine efficiencies around 8:30 am. In some implementations, such times may be chosen to trigger transitions from one heliostat field configuration to the next.

In some implementations, the arc 405 may change throughout the year. For example, in the northern hemisphere the Sun generally rises earlier in the morning, rises higher overhead at mid-day, and sets later at night in the summer than it does in the winter. As such, the specific configurations of the heliostats, the times at which they are used, the times at which configurations are changed over, or combinations of these and/or other factors may be used to anticipate the optimal organization of solar energy collection facilities throughout the year.

In some implementations, the configuration of a heliostat field may be substantially variable throughout the day. For example, rather than defining a finite number of configurations (e.g., the three illustrated by FIGS. 3A-3C), the receiver assignments of individual heliostats may be chosen and re-chosen dynamically throughout the day and year in order to optimize cosine efficiencies and/or totalized power reflected to the receivers.

Generally speaking, solar energy receivers can operate most efficiently for a given amount of received reflected solar energy. Furthermore, this operational efficiency may not be linearly proportional to the amount of reflected energy received. For example, in a combination of ten heliostats and two receivers on a Sunny day, five heliostats may reflect enough light to cause a single receiver to operate at or near its peak operational efficiency (e.g., 100%). Therefore the total output of the combination may be 200%. However, on an overcast day, for example where the amount of light is reduced by a third, the output of each may only be 40% (e.g., 80% total). In some implementations, a greater total output may be realized by completely shutting down some receivers and reassigning the corresponding heliostats to the remaining receivers. For example, on the aforementioned overcast day, one receiver may be sacrificed and all ten heliostats may be reassigned to the remaining receiver, causing enough light to be reflected to the remaining receiver to cause it to operate closer to peak operational efficiency. By operating a single receiver at 100% output, a greater total output may be realized than would be possible by running both at 40% output (e.g., 80% total).

Similarly, during times of the day when the sunlight is less intense, e.g., early morning and late afternoon, shutting in some receivers (i.e., not directing sunlight to some of the receivers) and redirecting the sunlight to the remaining activated receivers can improve the overall output of the solar energy collection facility. Referring again to FIG. 4, the area 420 schematically represents the time of day in the early morning when the sunlight is less intense at a given location for a given day. In the example shown, the sunrise is at approximately 6:00 am and the sunset is approximately 6:00 pm, although these times vary considerably with the particular location on Earth and time of year. During the time period represented by 420, some of the receivers can be closed-in (i.e., inactivated) and the heliostats can be assigned to the remaining receivers that are activated.

FIGS. 5A-5C show example configurations of heliostat assignments to concentrate available solar energy under varying daylight conditions. For example, in the morning configuration illustrated by FIG. 5A, a collection of receivers 505 have been closed-in (i.e., are inactivated) and the heliostats 515 have been assigned to reflect light upon a collection of receivers 510 that are activated. In the illustrated example, thirty six of the heliostats 515 have been assigned to each of the receivers 510. When compared to the morning configuration illustrated by FIG. 3A in which eighteen heliostats are assigned to each receiver, the configuration of FIG. 5A assigns a greater number (e.g., 36 of the heliostats 515 to the activated receivers 510 to compensate for the reduced amount of available morning sunlight during the time period 420.

Referring again to FIG. 4, during the time period represented by 430, the optimal heliostat assignment may be to activate all of the receivers, i.e., to re-activate the subset of receivers 505 that were closed-in during the time period 420. Referring now to FIG. 5B, heliostat assignments for a given time during the time period 430 are shown, in this particular example at a given time of mid-day. All of the receivers 505 and 510 are activated and the heliostats 515 have been reassigned such that eighteen of the heliostats 515 are assigned to each of the receivers 505 and 510. Various different heliostat assignments can be used during the time period 430 when all of the receivers are activated.

FIG. 5C represents the facility 500 in a late afternoon configuration, when the sunlight intensity has again decreased, and corresponds to a time during time period 440 shown in FIG. 4. Similar to the morning configuration shown in FIG. 5A, the receivers 505 have been sacrificed, and those receivers' 505 heliostats 515 have been reassigned to the receivers 510 to increase the total amount of solar energy reflected onto the receivers 510 to compensate for the reduced amount of available sunlight. In the illustrated example, thirty six of the heliostats 515 are assigned to each of the receivers 510. In the particular example shown, the same subset of receivers 505 was closed-in during the morning period 420 and the afternoon period 440. However, it should be understood that different subsets of receivers can be closed-in at different times of the day, and that the subset of closed-in receivers can be more or less than ½ of the total number of receivers and that the number of receivers closed-in can vary during the day as well. For example, during the time period 420, some of the closed-in receivers can be reactivated before others, allowing for a gradual start-up of all of the receivers in the field as the morning goes on. A similar gradual shutting-in effect can be taken toward the end of the day (i.e., during time period 440.

In the example described above, some of the receivers were closed-in during the certain times of the day based on predictable factors, i.e., the predicted sunlight intensity based on the location and time of year. In other implementations, some of the receivers can be closed-in during certain times of the day to account for unpredictable factors, e.g., changes in the weather that affect the sunlight intensity. For example, in a large solar energy collection facility, a passing cloud bank may reduce the amount of light that shines on a portion of the heliostat field. One possible way to reduce the impact of such unpredictable lighting conditions is by dynamically sacrificing some receivers and reassigning those receivers' heliostats among the remaining active receivers.

In some implementations, one or more sensors may be used to measure the amount of available sunlight at some or all of the receivers and/or heliostats in the facility 500. The measurements from the sensors can be used to dynamically re-assign the heliostats and potentially to close-in some of the receivers to account for reduced sunlight intensity. The measurements can be also used to later reactivate some or all of the closed-in receivers, for example, after a cloud bank has passed or clouds have dissipated.

FIG. 6 is a flow diagram of an example process 600 for assigning and reassigning a collection of heliostats among a collection of solar energy receivers in a solar energy collection facility. In some implementations, the process 600 may be used by the facilities 100, 300, and/or 500 to reconfigure the assignment of heliostats among receivers.

In some implementations, in addition to the Sun's daily and seasonal movement in the sky, other less predictable factors may affect the amount of solar energy collected by a receiver tower. For example, overcast skies, fog, rain, smoke, and airborne dust can variably reduce the amount of sunlight that shines upon a heliostat field. In another example, materials (e.g., snow, ice, dust, ash) or mechanical malfunctions can unexpectedly impede or prevent heliostats from properly reflecting solar energy until the heliostats can be cleaned or repaired. In such cases, the total amount of energy provided to a receiver may be less than what is needed to efficiently operate the receiver. In some implementations, the number of receivers, and the heliostats assigned to them, may be dynamically reconfigured in response to variations in the amount of solar energy received at the towers.

Initially, an initial heliostat assignment is imposed to the solar energy receiver towers (Step 610. In some implementations, the initial assignment may be determined using predictable patterns of the Sun's movement for various times of the day and/or various times of the year. In some implementations, the initial assignment may be determined using predicted weather conditions. For example, during the dark hours of the early morning a weather forecast may be used to anticipate that the sky will be cloudy and overcast at sunrise, and the heliostat assignment may be configured prior to sunrise in a way that may anticipate and/or compensate for the reduction in sunlight caused by the clouds at dawn.

Solar energy levels are monitored, e.g., at some or all of the active receiver towers (Step 620. If the solar energy levels at one or more receivers are less than a first threshold (“Yes” branch of Step 630, then a subset of the active receivers is inactivated (Step 640. A modified heliostat assignment is then imposed to the active receivers (Step 650. For example, when one or more receivers are receiving an insufficient amount of reflected light to operate efficiently, a controller may deactivate some receivers and direct the heliostats that were assigned to direct light toward those receivers to reflect their light to one of the remaining (e.g., active) receivers to increase the amount of solar energy being provided to those towers. Solar energy levels at some or all of the active receivers continue to be monitored (Step 620.

If, however, the solar energy levels at one or more receivers are not less than a first threshold (“No” branch of Step 630, then a second determination is made, i.e., whether the solar energy levels at some towers exceeds a second threshold (Step 660. If at the solar energy levels at some of the active receivers does not exceed a second threshold (“No” branch of Step 660, then monitoring of solar energy levels at the active receivers continues at (Step 620. However, if the solar energy levels at some of the active receivers exceeds the second threshold (“Yes” branch of Step 660, then some or all of the inactive receivers are reactivated (Step 670, and a modified heliostat assignment is imposed to the active receivers (Step 650. For example, when a receiver is provided with more solar energy than it can efficiently or effectively use, a controller may bring an additional receiver online to receive the excess energy. The controller may reassign one or more heliostats from the oversupplied receiver to the additional receiver by directing the reassigned heliostats to reflect their sunlight to the additional tower. Solar energy levels at the active receivers continue to be monitored (Step 620.

FIGS. 7A-7B illustrate example configurations of heliostats and solar energy receivers in all north field configuration 700. In some implementations, heliostats located on roughly the opposite side of a solar energy receiver from the Sun may exhibit lower cosine losses than those seen at heliostats located between the receivers and the Sun. For example, heliostats located opposite of the Sun (e.g., in north fields) may reflect the incoming solar energy to the receiver at angles that are closer to their surface normals as compared to heliostats located between the receiver and the Sun (e.g., in south fields).

In some implementations, the overall cosine losses of a heliostat field may be reduced by arranging heliostats and receivers, such that a majority or substantial entirety of the heliostats are configured in north field arrangements (e.g. opposite the receivers from the Sun). Furthermore, by dynamically altering the assignment of heliostats to receivers during the day, still lower cosine losses may be achieved. Referring to FIG. 7A, the configuration 700 is illustrated in a morning arrangement. A collection of heliostats 705 is assigned to a collection of solar energy receivers 710 that are located between the heliostats 705 and the incoming sunlight, the general direction of which is indicated by a collection of arrows 730. Similarly, a collection of heliostats 715 are assigned to a collection of solar energy receivers 720 that are also located between the heliostats 715 and the incoming sunlight.

Since the Sun rises in the east, the morning sunlight falls upon the configuration 700 from a southeast direction as indicated by the arrows 730. In addition to taking advantage of the lower cosine losses offered by a substantially north field arrangement of the heliostats 705, 715, even lower cosine losses may be achieved by dynamically reconfiguring the assignment of the heliostats 705, 715 to the receivers 710, 720 as the day progresses. For example, as illustrated by FIG. 7A, the heliostats 705, 715 are assigned in groups that are roughly opposite the receivers 710, 720, from the incoming sunlight. For example, a group of heliostats 740 is configured in a modified north field configuration behind, and assigned to reflect their solar energy to, a receiver 750.

Referring now to FIG. 7B, the configuration 700 is assigned in an afternoon arrangement. The arrows 730 generally indicate the southwest origin of the afternoon sunlight. The configuration 700 has been reorganized from its morning arrangement to better align and group the heliostats 705, 715 with the receivers 710, 720 to reduce cosine losses. In the illustrated example, the receiver 750 has a group of heliostats 760 assigned to reflect light to it, and the group 760 is organized in a modified north field configuration where the heliostats 705 in the group 760 are located roughly opposite the receiver 750 from the incoming sunlight.

FIG. 8 is a block diagram of an example heliostat field system 800. In various implementations, the system 800 may be the facilities 100, 300, 500, or 700. The system 800 includes a collection of heliostats 805 arranged as a heliostat field 810. In some implementations, the heliostat field 810 may represent a so-called north field or south field of a solar energy collection facility. The system shown is simplified for illustrative purposes and may include many dozens, hundreds or even thousands of heliostats 805 and many dozens, hundreds or even thousands of receiver towers.

The heliostats 805 are each able to vary the direction in which their one or more reflective surfaces are pointing. As such, the heliostats 805 can be pitched and angled so as to selectably reflect incoming sunlight, represented by arrows 815, to either a solar energy receiver tower 820 a or a solar energy receiver tower 820 b. Arrows 825 represent the reflected sunlight. The solar energy receiver towers 820 a and 820 b each include a solar energy receiver 830 a and 830 b respectively. The solar energy receivers 830 a, 830 b are configured to receive solar rays reflected by the heliostats 805. The heliostats' 805 pitches and angles can be varied throughout the day to track the Sun as it appears to move across the daytime sky in order to maintain their reflective relationship with a selected one of the receivers 830 a, 830 b to which they are assigned to direct solar rays.

The heliostats 805 are communicably connected to an assignment control system 835, e.g., by communication lines 840. In some implementations, the communication lines 840 may conduct power to the heliostats 805 (e.g., to energize their pitch and angle mechanisms). In some implementations, the communication lines 840 may be supplemented or replaced by wireless communication links between the heliostats 805 and the assignment controller 835. The assignment control system 835 communicates with the heliostats 805 to assign each of the heliostats 805 to direct solar rays to two or more different receiver towers (e.g., the receiver towers 820 a, 820 b) at two or more different times of day, wherein each of the heliostats 805 directs solar rays to one of the receiver towers 820 a, 820 b at a time. The assigning can be based on estimated efficiencies of the heliostats 805 in directing solar rays to the receiver towers 820 a, 820 b determined for a plurality of times of day, and/or based on actually efficiencies.

In some implementations, the assignment control system 835 may be configured to assign the heliostats 805 to direct solar rays to a subset of the receiver towers 820 a, 820 b, such that the remaining receiver towers are closed-in, e.g., based on estimated levels of solar ray intensity at different times of the day. For example, in the early morning or late afternoon the assignment control system 835 may reassign the heliostats 805 normally assigned to the receiver tower 820 b to the receiver tower 820 a. Optionally, the assignment control system 835 can command the receiver tower 820 b to close-in. In some implementations, a receiver tower closes in by closing shutters to block a receiver face (which may include a receiver aperture, for example, for a cavity style receiver) that is adapted to receive the solar rays. In other implementations, the receiver tower doesn't actually undergo a change at all, other than that no heliostats are assigned to direct solar rays to the receiver. In some implementations, an engine coupled to the receiver to generate power is powered down. Other steps to deactivate a receiver can be taken, and these are but some examples. In some implementations, the assignment control system 835 may be configured to assign the heliostats 805 to direct solar rays to the receiver towers 820 a, 820 b, such that the closed-in receiver towers are reactivated, based on the estimated levels of solar ray intensity at different times of the day.

A heliostat tracking control system 845 is configured to control the positioning of one or more reflective surfaces included on each of the heliostats 805 based on a position of the Sun and which of the receiver towers 820 a, 820 b the heliostat 805 is assigned to direct solar rays toward. In some implementations, the controller 845 may substantially control the pitch and angle of the heliostats 805 to control the direction in which their light is reflected. In some implementations, the heliostat tracking control system 845 is implemented as a controller at each of the individual heliostats 805. That is, the heliostats 805 may include processors that substantially independently determine and control the pitch and angle of the heliostats reflectors based on an assignment sent from the assignment control system 835.

A solar energy level monitoring system 850 is configured to monitor, for each of at least some of the receiver towers 820 a, 820 b, a level of solar energy absorbed by the receivers 830 a, 830 b, mounted on the receiver towers 820 a, 820 b. In some implementations, the solar energy level monitoring system 850 further includes (or alternatively includes) a collection of solar energy sensors 855 that sense the intensity and/or direction of incoming sunlight.

In some implementations, the assignment control system 835 may be configured to assign the heliostats 805 to direct solar rays to a subset of the receiver towers 820 a, 820 b, such that the remaining receiver towers are closed-in when at least some of the monitored levels of solar energy are below a predetermined first threshold value. For example, the solar energy monitoring system 850 may detect that received solar energy levels at both of the receivers 830 a and 830 b is below a predetermined threshold value (e.g., based on the efficiencies of the receivers 830 a, 830 b for different amounts of received solar energy), and the assignment control system 835 may use this information to close-in the receiver tower 820 a and reassign additional ones of the heliostats 805 to the receiver tower 820 b. In another example, the solar energy monitoring system 850 may monitor the solar energy sensors 855 and determine that the intensity of incoming sunlight has fallen below a threshold value (e.g., a cloud bank is reducing the amount of sunlight shining on some of the heliostats 805, and the assignment control system 835 may use this information to reassign the heliostats 805 among the receiver towers 820 a, 820 b.

The assignment control system 835 may, in some implementations, assign the heliostats 805 to direct solar rays to the receiver towers 820 a, 820 b, such that the closed-in receiver towers are reactivated, when at least some of the monitored levels of solar energy are above a predetermined threshold value (e.g., inactive receiver towers 820 a, 820 b, may be reactivated to take advantage of additional available solar power). In some implementations, the solar energy sensors 855 may sense and/or track the position of the Sun, and provide that positional information to the heliostat tracking controller 845. In some implementations, a number of the solar energy sensors 855 may be located throughout the heliostat field 810 to provide the solar energy monitoring system 850 with solar energy intensity information for various locations across the heliostat field 810.

To determine various assignments of the heliostats 805 and the receiver towers 820 a, 820 b to reduce inefficiencies, e.g., cosine losses, in some implementations the assignment control system 835 may use the information it receives from the solar energy monitoring system 850, along with time and date information, weather forecast information, astronomical information (e.g., seasonal arcs of the Sun, sunrise and sunset information, predictions for solar eclipses), and combinations of these and/or other types of information. In some implementations, the assignment controller 835 may use a combination of time and date information along with astronomical information to determine an initial assignment. For example, just before dawn, the assignment controller 835 may prepare the system 800 to collect the morning sunlight by assigning the heliostats 805 to the receivers 820 a, 820 b in a morning configuration such as that illustrated in FIG. 3A.

In some implementations, the assignment controller 835 may use weather information, forecasts, sensed solar energy intensity information (e.g., from the sensors 855, received solar energy information (e.g., from the receivers 830 a, 830 b), or combinations of these and or other types of information that may be variable or substantially unpredictable to dynamically modify the assignments of the heliostats 805 and the receiver towers 820 a, 820 b. For example, smoke from a forest fire may unexpectedly block out a portion of the light that would normally shine on the heliostats 805, and the assignment controller 835 may respond to the reduced amount of light by shutting in the receiver tower 820 a and signaling the heliostats 805 normally assigned to the receiver tower 820 a to consolidate their reflected light upon the remaining receiver tower 820 b. In another example, a malfunction may cause the assignment controller 835 and/or the heliostat tracking controller 845 to lose communication with and control of parts of the heliostat field 810, rendering some of the heliostats 805 in that field 810 substantially unable to maintain their reflective relationship with the receivers 830 a, 830 b. The assignment controller 835 may respond by consolidating the solar energy reflected by the remaining, operational heliostats 805 onto a single one of the receivers 820 a or 820 b.

In some implementations, the heliostats 805 focus the Sun's energy onto receivers 830 a, 830 b to heat a working fluid, e.g., water, air or molten salt. The working fluid can travel through a heat exchanger 860 to heat water, produce steam, and then generate electricity through a turbine 870 connected to a generator 880. In some implementations, the heliostats 805 focus the Sun's energy onto receivers 830 a, 830 b to heat air or another gas. The heated gas is then expanded through the turbine 870, which turns a shaft to drive the generator 880. The electricity can be conducted, e.g., by wires 890, to a utility grid, or some other point where the electricity can be distributed or consumed. In some implementations, some of the electricity may be consumed by the system 800 itself, e.g., by the assignment controller 835 and/or by the heliostats 805 in order to move to track the Sun and/or to move based on a new receiver assignment. In some implementations, the heat exchanger 860, turbine 870 and generator 880 can be implemented on a per-receiver tower basis and can be included at each receiver tower. Alternatively, a heat exchanger, turbine and generator can be positioned to service a subset of the receiver towers.

In some implementations, the receiver can be configured to move, e.g., rotating about a vertical axis, translating or both. Moving the receiver may enhance solar energy received at the receiver and reduce cosine losses. FIG. 9 is a schematic representation of an example receiver 904 mounted on a receiver tower 902 in a heliostat field that includes multiple heliostats 918 and 920. Although six heliostats are shown, in practice, many more heliostats can be used in a field, in some examples, in the hundreds or even thousands. The receiver 904 includes a receiver face 905 that is configured to receive solar energy reflected from multiple heliostats that are assigned to direct solar rays to the receiver 904. It should be understood that in some implementations, the receiver face 905 includes a surface that is configured to receive the solar energy, and in other implementations, e.g., a cavity receiver, the receiver face 905 includes an aperture behind which is formed a cavity. The surface of the cavity receives the solar rays that are incident on the aperture formed in the receiver face 905. Other configurations of receiver are possible. The receiver 904 can be mounted on the tower 902 with a mount 906 that is configured to allow the receiver 904 to rotate about the axis 908 in the directions indicated by the arrow 910, e.g., with a thrust bearing although other configurations of mount can be used. The receiver face 905 can thereby be selectively reoriented to face in different directions.

By way of example, in the Northern hemisphere, during the morning when the Sun is in the east, it may be more efficient to have the heliostats 920 that are positioned west of the receiver 904 direct solar energy to the receiver 904, and the receiver face 905 can be positioned to face toward the heliostats 920. In the afternoon, when the Sun is in the west, the receiver face 905 can be repositioned to face toward the heliostats 918 that are positioned to the east of the receiver 904. Therefore, during the course of the day, the heliostats that are assigned to direct solar energy to the receiver 904 can change along with the direction that the receiver face 905 is facing. Other heliostat assignments are possible, for example, heliostats can be positioned to the north and south of the receiver 904 and the heliostat assignment can be changed throughout the day based on position of the Sun and/or environmental conditions, along with movement of the receiver face 905, so as to optimize the solar energy received by the receiver face 905. In some implementation, the heliostats that are assigned to direct solar energy to the receiver 904 do not change, but the position of the receiver face 905 does change to optimize the solar energy received.

In some implementations, the receiver 904 can also be pivoted about a horizontal axis to change the elevation of the receiver face 905. That is, the position of the receiver face 905 can be adjusted to point downwardly or upwardly.

The receiver tower 902 is typically secured to the ground 916. For example, the tower 902 can be a pole that is mounted several feet down into the Earth, to provide for a secure and rigid attachment. In some implementations, the tower 902 itself can be movable. The tower 902 can be mounted on a mounting assembly 912 that is configured to rotate the tower 902 about the axis 908 in the directions of the arrow 910. In some implementations, the mounting assembly 912 can be configured to translate the tower 902 to move to different positions within the field, for example, in the direction of the arrows 914, although other directions are also possible. The entire tower 902 can thereby be moved to reposition the receiver 904 and to reorient the receiver face 905. The tower 902 can be moved to better position the receiver face 905 with respect to either a fixed set of heliostats that are assigned to direct solar energy to the receiver face 905, or with respect to a dynamically assigned set of heliostats, i.e., a set of heliostats that can change over time to accommodate for the position of the Sun and/or environmental conditions.

In some implementations, a controller that is either local at the receiver 904 or is remote to the receiver 904, can provide signals to instruct one or more actuator assemblies to move the receiver 904 relative to the tower 902 (e.g., rotate about axis 908 and/or to move the tower 902 relative to the ground 916. The adjustments to the position of the receiver face throughout the day can occur at predetermined intervals or can be continuous. In some implementations, communications between the controller and the one or more actuator assemblies can be over a wired communications system, e.g., an Ethernet network, an I2C network, an RS232/RS422 connection, or other appropriate wired connection. In another example, the communications ca be over a fiber optic connection. In another example, the communications can be over a wireless network, e.g., a wireless Ethernet (e.g., 802.11 network, a ZigBee network, a cellular network, or other appropriate wireless network. In some implementations, multiple estimated efficiencies of the receiver in receiving solar rays at the receiver face from the set of heliostats at multiple different times of day can be determined. The controller can control the adjustment of the position of the receiver face based on the estimated efficiencies for the receiver at the multiple times of day. In some implementations, the controller can control adjustment of the position of the receiver face based on a level of solar energy absorbed by the receiver, which can be measured by one or more sensors positioned at the receiver or elsewhere. In some implementations, the controller can control adjustment of the position of the receiver face based on estimated levels of solar ray intensity at different times of the day and/or on different days of the year. In some implementations, the controller can control adjustment of the position of the receiver face based on the azimuth direction of the Sun. That is, the receiver face can be adjusted to approximately the same direction as the azimuth direction of the Sun. The adjustments can be based on the estimated azimuth direction of the Sun (e.g., estimated based on the location of the receiver, the time of day and the time of year), or based on the actual azimuth direction of the Sun (e.g., which can be determined based on one or more sensor measurements).

Referring again to FIG. 8, in some implementations, the controller can be the assignment control 835, which can be further configured to control movement of one or more receivers, e.g., receivers 830 a and 803 b, which can be configured in a manner similar to receiver 904 shown in FIG. 9. Movement of the receivers can be based on information from one or more sensors, e.g., sensors 855 and/or sensors positioned on the receivers, and/or based on predictable conditions, such as the position of the Sun and/or based on unpredictable conditions, such as the weather.

An actuator assembly can be positioned at or near the mount 906 to drive the receiver 904 about the axis 908. A second actuator assembly can be positioned at or near the mounting assembly 912 to move the receiver tower 902 either in rotation and/or relative to the ground 916. The actuator assemblies can include one or more motors, bearings and/or wheels or tracks, depending on the type of movement being imposed. Other configurations of actuator assembly are possible, and the ones described are illustrative and non-limiting.

In some implementations, a cost-benefit analysis can be used when determining whether to change a heliostat-receiver assignment. Heliostats generally do not have fast rates of movement, as they move to track the Sun and therefore move at a relatively low rate of speed throughout the day. As such, the time to reorient a heliostat to point toward a different receiver may take several minutes, for example, 15 minutes. While the heliostat is in transition during the reorientation process, there is a loss of energy from that heliostat, i.e., a cost is incurred. In determining whether to re-orient a heliostat to point toward a different receiver, the expected benefit of the reorientation can be compared to the cost of reorientation. The benefit can be a function of the duration of sunlight remaining in the day, the time the heliostat will be pointed toward the new receiver and the sunlight expected to be received over the day.

The following is an illustrative example of a cost-benefit analysis. In the example, the benefit of a reorientation of the heliostat is expected to be a 10% improvement over the current output being 900 watts per square meter. At a 25% conversion efficiency for two hours, the benefit can be calculated as follows:

Benefit=0.10×900 Watts/m²×2 hours=45 Watt hours

The cost (i.e., the energy not captured) of reorienting the heliostat can be calculated as below, where in this example the time to move the heliostat is 15 minutes.

Cost=0.25×900 Watts/m²×15/60 hr=56.25 Watt hours

In this example, the cost exceeds the benefit (i.e., 56.25 exceeds 45, and therefore based on the cost-benefit analysis, it does not make sense to reorient the heliostat. A similar evaluation can be used to decide whether to move flux from a first receiver, e.g., receiver A, to either of two other receivers, e.g., receivers B and C. Based on the evaluation, the heliostat may be reoriented to point toward receiver C because the heliostat can be orientated to point toward receiver C faster than reorienting to point toward receiver B, even if receiver C's power output is not as optimal as receiver B (although both have better power output than receiver A).

In some implementations, flux distribution at the receivers can be used in determining whether to reorient heliostats to point toward particular receivers. Assigning a heliostat to point toward a first receiver, e.g., receiver A, may have beneficial (or less detrimental) effects than if the heliostat is pointing toward a second receiver, e.g., receiver B. Receivers can be a costly point of failure in a solar thermal power conversion system. For example, receiver melting can be an expensive failure that is preferably avoided. Receiver life is a function of fatigue, and fatigue is worsened by significant stresses that can be caused by thermal expansion and/or differential thermal expansion. The flux distribution within a receiver impacts the temperature of receiver materials, which impacts absolute temperatures and temperature gradients. For example, a receiver with large flux variations can have uneven heating. Such a receiver would therefore have to be operated at a lower working fluid temperature operating point, to avoid peak temperatures approaching melting or temperature profiles that disproportionately shorten receiver life due to fatigue. Operating a thermal receiver at a lower working fluid temperature will lower the efficiency of the power conversion system. Accordingly, it may be preferable to pull some of the flux (e.g., the most peaky flux) out of receiver B and distribute this heat into one or more other receivers, e.g., receiver A. Doing so may allow the receiver B to operate at a higher working fluid temperature, and therefore produce output power more efficiently. The additional flux thereby added to the other receiver(s), e.g., receiver A, can add some power output as well, with less impact on working fluid temperature requirements. As a result, the total electrical power output of the field can increase.

FIG. 10 is a schematic diagram of an example of a generic computer system 1000. The system 1000 can be used for the operations described in association with the process 600 according to one implementation. For example, the system 1000 may be included in either or all of the controllers 835, 845, the sensors 855, or in the heliostats 110, 120, 302, 515, 705, 715, and 805.

The system 1000 includes a processor 1010, a memory 1020, a storage device 1030, and an input/output device 1040. Each of the components 1010, 1020, 1030, and 1040 are interconnected using a system bus 1050. The processor 1010 is capable of processing instructions for execution within the system 1000. In one implementation, the processor 1010 is a single-threaded processor. In another implementation, the processor 1010 is a multi-threaded processor. The processor 1010 is capable of processing instructions stored in the memory 1020 or on the storage device 1030 to display graphical information for a user interface on the input/output device 1040.

The memory 1020 stores information within the system 1000. In one implementation, the memory 1020 is a computer-readable medium. In one implementation, the memory 1020 is a volatile memory unit. In another implementation, the memory 1020 is a non-volatile memory unit.

The storage device 1030 is capable of providing mass storage for the system 1000. In one implementation, the storage device 1030 is a computer-readable medium. In various different implementations, the storage device 1030 may be a floppy disk device, a hard disk device, an optical disk device, or a tape device.

The input/output device 1040 provides input/output operations for the system 1000. In one implementation, the input/output device 1040 includes a keyboard and/or pointing device. In another implementation, the input/output device 1040 includes a display unit for displaying graphical user interfaces.

Various implementations of the systems and techniques described here may be realized in digital electronic circuitry, integrated circuitry, specially designed ASICs (application specific integrated circuits), computer hardware, firmware, software, and/or combinations thereof. These various implementations may include implementation in one or more computer programs that are executable and/or interpretable on a programmable system including at least one programmable processor, which may be special or general purpose, coupled to receive data and instructions from, and to transmit data and instructions to, a storage system, at least one input device, and at least one output device.

These computer programs (also known as programs, software, software applications or code) include machine instructions for a programmable processor, and may be implemented in a high-level procedural and/or object-oriented programming language, and/or in assembly/machine language. As used herein, the terms “machine-readable medium” “computer-readable medium” refers to any computer program product, apparatus and/or device (e.g., magnetic discs, optical disks, memory, Programmable Logic Devices (PLDs)) used to provide machine instructions and/or data to a programmable processor, including a machine-readable medium that receives machine instructions as a machine-readable signal. The term “machine-readable signal” refers to any signal used to provide machine instructions and/or data to a programmable processor.

To provide for interaction with a user, the systems and techniques described here may be implemented on a computer having a display device (e.g., a CRT (cathode ray tube) or LCD (liquid crystal display) monitor) for displaying information to the user and a keyboard and a pointing device (e.g., a mouse or a trackball) by which the user may provide input to the computer. Other kinds of devices may be used to provide for interaction with a user as well; for example, feedback provided to the user may be any form of sensory feedback (e.g., visual feedback, auditory feedback, or tactile feedback); and input from the user may be received in any form, including acoustic, speech, or tactile input.

The systems and techniques described here may be implemented in a computing system that includes a back end component (e.g., as a data server), or that includes a middleware component (e.g., an application server), or that includes a front end component (e.g., a client computer having a graphical user interface or a Web browser through which a user may interact with an implementation of the systems and techniques described here), or any combination of such back end, middleware, or front end components. The components of the system may be interconnected by any form or medium of digital data communication (e.g., a communication network). Examples of communication networks include a local area network (“LAN”), a wide area network (“WAN”), and the Internet.

The computing system may include clients and servers. A client and server are generally remote from each other and typically interact through a communication network. The relationship of client and server arises by virtue of computer programs running on the respective computers and having a client-server relationship to each other.

While this specification contains many specific implementation details, these should not be construed as limitations on the scope of any invention or of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments of particular inventions. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the embodiments described above should not be understood as requiring such separation in all embodiments, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products.

A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention.

In addition, the logic flows depicted in the figures do not require the particular order shown, or sequential order, to achieve desirable results. In addition, other steps may be provided, or steps may be eliminated, from the described flows, and other components may be added to, or removed from, the described systems. Accordingly, other embodiments are within the scope of the following claims. 

1. A method for operating a multi-tower heliostat field, comprising: for each heliostat of a plurality of heliostats that are arranged about a plurality of receiver towers, determining a plurality of estimated efficiencies of the heliostat in directing solar rays to two or more receiver towers at a plurality of times of day, wherein each receiver tower has a receiver mounted to the tower that is configured to receive solar rays reflected from heliostats included in the plurality of heliostats; assigning each heliostat to direct solar rays to two or more different receiver towers at two or more different times of day, wherein each heliostat directs solar rays to one receiver tower at a time and the assigning is based on the determined estimated efficiencies for the heliostat at the plurality of times of day.
 2. The method of claim 1, wherein: determining a plurality of estimated efficiencies of each heliostat further comprises determining a plurality of estimated efficiencies of the heliostat in directing solar rays to the two or more receiver towers at a plurality of times of day on a plurality of days of a year; and assigning each heliostat to direct solar rays to two or more different receiver towers is further based on the determined estimated efficiencies at the plurality of times of day on the plurality of days of the year.
 3. The method of claim 1, wherein assigning each heliostat to direct solar rays to two or more different receiver towers comprising assigning each heliostat to direct solar rays to two or more different receiver towers that are each positioned south of the heliostat for a heliostat field in the northern hemisphere and that are each positioned north of the heliostat for a heliostat field in the southern hemisphere.
 4. The method of claim 1, further comprising: for each heliostat, controlling positioning of one or more reflective surfaces included on the heliostat based on a position of the Sun and which receiver tower the heliostat is assigned to direct solar rays toward.
 5. The method of claim 1, wherein each receiver is configured to receive solar rays at a receiver face, the method further comprising: selectively moving one or more receivers mounted to one or more receiver towers to adjust positions of the one or more receiver faces.
 6. The method of claim 5, wherein the positions of the one or more receiver faces are adjusted in accordance with positions of heliostats assigned to direct solar rays to the one or more receivers corresponding to the receiver faces.
 7. The method of claim 5, wherein the positions of the one or more receiver faces are adjusted based on an azimuth direction of the Sun.
 8. The method of claim 5, wherein moving one or more receivers comprises rotating the one or more receivers about a vertical axis to adjust the positions of the one or more receiver faces.
 9. The method of claim 5, wherein moving one or more receivers comprises rotating the one or more receivers about a horizontal axis to adjust the positions of the one or more receiver faces.
 10. The method of claim 5, wherein moving one or more receivers comprises moving the receiver towers that correspond to the one or more receivers to adjust the positions of the one or more receiver faces.
 11. The method of claim 1, wherein assigning each heliostat to direct solar rays to two or more different receiver towers at two or more different times of day further comprises: for a particular heliostat, determining whether a benefit of reassigning the particular heliostat from a first receiver tower to a second receiver tower outweighs a cost of the reassigning, wherein the assigning is based at least in part on the determination.
 12. The method of claim 1, wherein assigning each heliostat to direct solar rays to two or more different receiver towers at two or more different times of day further comprises: for a particular heliostat, determining flux distributions over surfaces of a first receiver and a second receiver when the heliostat is assigned to direct solar rays to first and second receiver towers that correspond to the first and second receivers, wherein the assigning is based at least in part on the determination of flux distribution.
 13. A method for operating a multi-tower heliostat field, comprising: for each receiver tower of a plurality of receiver towers about which are arranged a plurality of heliostats, assigning a set of heliostats to direct solar rays to the receiver tower, wherein each receiver tower has a receiver mounted to the tower that is configured to receive solar rays reflected from heliostats included in the plurality of heliostats; and for each receiver tower, based on a level of solar energy absorbed by the receiver mounted to the tower, reassigning which heliostats from the plurality of heliostats are included in the set of heliostats that are assigned to direct solar rays to the receiver tower.
 14. The method of claim 13, further comprising: for each receiver tower, monitoring the level of solar energy absorbed by the receiver mounted to the tower during daylight hours.
 15. The method of claim 14, wherein when the monitored level of solar energy drops below a first predetermined threshold value for one or more of the receiver towers, closing in a subset of the receiver towers and reassigning the heliostats included in the sets of heliostats assigned to direct solar rays to the closed-in subset of receiver towers to different receiver towers included in the plurality of receiver towers.
 16. The method of claim 15, wherein when the monitored level of solar energy rises above a second predetermined threshold value for one or more of the receiver towers, re-activating one or more of the closed-in receiver towers and reassigning at least some of the heliostats to direct solar rays to the reactivated one or more receiver towers.
 17. The method of claim 13, further comprising: for each heliostat, controlling positioning of one or more reflective surfaces included on the heliostat based on a position of the Sun and which receiver tower the heliostat is assigned to direct solar rays toward.
 18. The method of claim 13, wherein each receiver is configured to receive solar rays at a receiver face, the method further comprising: selectively moving one or more receivers mounted to one or more receiver towers to adjust positions of the one or more receiver faces.
 19. The method of claim 18, wherein the positions of the one or more receiver faces are adjusted in accordance with positions of heliostats assigned to direct solar rays to the one or more receivers corresponding to the receiver faces.
 20. The method of claim 18, wherein the positions of the one or more receiver faces are adjusted based on an azimuth direction of the Sun.
 21. The method of claim 18, wherein moving one or more receivers comprises rotating the one or more receivers about a vertical axis to adjust the positions of the one or more receiver faces.
 22. The method of claim 18, wherein moving one or more receivers comprises rotating the one or more receivers about a horizontal axis to adjust the positions of the one or more receiver faces.
 23. The method of claim 18, wherein moving one or more receivers comprises moving the receiver towers that correspond to the one or more receivers to adjust the positions of the one or more receiver faces.
 24. The method of claim 13, wherein reassigning which heliostats from the plurality of heliostats are included in the set of heliostats that are assigned to direct solar rays to the receiver tower, further comprises: determining whether a benefit of reassigning the heliostats from a first receiver tower to a second receiver tower outweighs a cost of the reassigning, wherein the reassigning is based at least in part on the determination.
 25. The method of claim 13, wherein reassigning which heliostats from the plurality of heliostats are included in the set of heliostats that are assigned to direct solar rays to the receiver tower, further comprises: determining flux distributions over surfaces of a first receiver and a second receiver the heliostats are assigned to direct solar rays to first and second receiver towers that correspond to the first and second receivers, wherein the reassigning is based at least in part on the determination of flux distribution.
 26. A method for operating a multi-tower heliostat field, comprising: for each receiver tower of a plurality of receiver towers about which are arranged a plurality of heliostats, assigning a set of heliostats to direct solar rays to the receiver tower, wherein each receiver tower has a receiver mounted to the tower that is configured to receive solar rays reflected from heliostats included in the plurality of heliostats; and based on estimated levels of solar ray intensity at different times of the day, closing in a subset of the receiver towers during one or more time periods a day and reassigning the heliostats included in the sets of heliostats assigned to the closed-in subset of receiver towers to different receiver towers included in the plurality of receiver towers during those time periods.
 27. The method of claim 26, wherein closing in a subset of the receiver towers and reassigning the heliostats is further based on estimated levels of solar ray intensity at different times of the day and at different times of the year.
 28. The method of claim 26, further comprising: for each heliostat, controlling positioning of one or more reflective surfaces included on the heliostat based on a position of the Sun and which receiver tower the heliostat is assigned to direct solar rays toward.
 29. The method of claim 26, wherein each receiver is configured to receive solar rays at a receiver face, the method further comprising selectively moving one or more receivers mounted to one or more receiver towers to adjust positions of the one or more receiver faces.
 30. The method of claim 29, wherein the positions of the one or more receiver faces are adjusted in accordance with positions of heliostats assigned to direct solar rays to the one or more receivers corresponding to the receiver faces.
 31. The method of claim 29, wherein the positions of the one or more receiver faces are adjusted based on an azimuth direction of the Sun.
 32. The method of claim 29, wherein moving one or more receivers comprises rotating the one or more receivers about a vertical axis to adjust the positions of the one or more receiver faces.
 33. The method of claim 29, wherein moving one or more receivers comprises rotating the one or more receivers about a horizontal axis to adjust the positions of the one or more receiver faces.
 34. The method of claim 29, wherein moving one or more receivers comprises moving the receiver towers that correspond to the one or more receivers to adjust the positions of the one or more receiver faces.
 35. A heliostat field system comprising: a plurality of heliostats; a plurality of receiver towers, wherein each receiver tower has a receiver mounted to the tower that is configured to receive solar rays reflected from a set of heliostats included in the plurality of heliostat, wherein the set of heliostats are assigned to direct solar rays to the receiver tower; and an assignment control system configured to assign each heliostat to direct solar rays to two or more different receiver towers at two or more different times of day, wherein each heliostat directs solar rays to one receiver tower at a time and the assigning is based on estimated efficiencies of the heliostat in directing solar rays to the two or more receiver towers determined for a plurality of times of day.
 36. The heliostat field system of claim 35, further comprising: a heliostat tracking control system configured to control, for each of the plurality of heliostats, positioning of one or more reflective surfaces included on each heliostat based on a position of the Sun and which receiver tower the heliostat is assigned to direct solar rays toward.
 37. The system of claim 35, wherein the assignment control system is further configured to assign the plurality of heliostats to direct solar rays to a subset of the receiver towers, such that the remaining receiver towers are closed-in, based on estimated levels of solar ray intensity at different times of the day.
 38. The system of claim 35, wherein the assignment control system is further configured to assign the plurality of heliostats to direct solar rays to the plurality of receiver towers, such that the closed-in receiver towers are reactivated, based on the estimated levels of solar ray intensity at different times of the day.
 39. The system of claim 35, further comprising: a solar energy level monitoring system configured to monitor, for each of at least some of the receiver towers, a level of solar energy absorbed by the receiver mounted on the receiver tower; wherein the assignment control system is further configured to assign each heliostat to direct solar rays to two or more different receiver towers based on the monitored levels of solar energy.
 40. The system of claim 39, wherein the assignment control system is further configured to assign the plurality of heliostats to direct solar rays to a subset of the receiver towers, such that the remaining receiver towers are closed-in, when at least some of the monitored levels of solar energy are below a predetermined first threshold value.
 41. The system of claim 40, wherein the assignment control system is further configured to assign the plurality of heliostats to direct solar rays to the plurality of receiver towers, such that the closed-in receiver towers are reactivated, when at least some of the monitored levels of solar energy are above a predetermined second threshold value.
 42. The heliostat field system of claim 35, wherein at least one or more of the receiver towers includes a receiver that is repositionable such that a face of the receiver that receives solar rays can be repositioned.
 43. The heliostat field system of claim 42, wherein the face of the receiver is repositionable in accordance with positions of the heliostats assigned to direct solar rays to the receiver.
 44. The heliostat field system of claim 42, wherein the face of the receiver is repositionable based on an azimuth direction of the Sun.
 45. The heliostat field system of claim 42, wherein the assignment control system is further configured to control repositioning of the face of the at least one receiver.
 46. The heliostat field system of claim 42, wherein the at least one receiver is configured to rotate about a vertical axis to adjust the position of the receiver face.
 47. The heliostat field system of claim 42, wherein the at least one receiver is configured to rotate about a horizontal axis to adjust the position of the receiver face.
 48. The heliostat field system of claim 42, wherein the receiver tower on which is mounted the at least one receiver is a receiver tower that is configured to move.
 49. The heliostat field system of claim 35, wherein the assignment control system is further configured to: for a particular heliostat, determine whether a benefit of reassigning the particular heliostat from a first receiver tower to a second receiver tower outweighs a cost of the reassigning, wherein the assigning is based at least in part on the determination.
 50. The heliostat field system of claim 35, wherein the assignment control system is further configured to: for a particular heliostat, determine flux distributions over surfaces of a first receiver and a second receiver when the heliostat is assigned to direct solar rays to first and second receiver towers that correspond to the first and second receivers, wherein the assigning is based at least in part on the determination of flux distribution.
 51. A method for operating a multi-tower heliostat field, comprising: for each receiver mounted to a receiver tower of a plurality of receiver towers about which a plurality of heliostats are arranged, assigning a set of multiple heliostats to direct solar rays to a receiver face of the receiver; and adjusting a position of the receiver face at a plurality of times throughout the coarse of a day.
 52. The method of claim 51, wherein adjusting the position of the receiver face is based on an azimuth direction of the Sun at the plurality of times throughout the coarse of the day.
 53. The method of claim 51, wherein adjusting the position of the receiver face comprises rotating the receiver about a vertical axis.
 54. The method of claim 51, wherein adjusting the position of the receiver face comprises rotating the receiver about a horizontal axis.
 55. The method of claim 51, wherein adjusting the position of the receiver face comprises moving the receiver tower.
 56. The method of claim 51, further comprising: for each receiver, determining a plurality of estimated efficiencies of the receiver in receiving solar rays at the receiver face from the set of multiple heliostats at a plurality of times of day; wherein adjusting the position of the receiver face is based on the estimated efficiencies for the receiver at the plurality of times of day.
 57. The method of claim 51, wherein adjusting the position of the receiver face is based on a level of solar energy absorbed by the receiver.
 58. The method of claim 51, wherein adjusting the position of the receiver face is based on estimated levels of solar ray intensity at different times of the day.
 59. The method of claim 51, further comprising: for each heliostat of the plurality of heliostats, determining a plurality of estimated efficiencies of the heliostat in directing solar rays to two or more receiver towers of the plurality of receiver towers at a plurality of times of day; and wherein assigning a set of multiple heliostats to direct solar rays to a receiver face of the receiver comprises assigning a plurality of sets of multiple heliostats to direct solar rays to the receiver face at a plurality of times of the day based on the determined estimated efficiencies of the plurality of heliostats. 